Find 6 three-bedroom apartments for rent in O'Fallon, IL. A three-bedroom apartment is a unit with three distinct bedrooms that share a common area and kitchen. Depending on the floor plan, each room may share or have its own bathroom. If roommates split the cost of a three-bedroom rental, it can be a very affordable option. Additionally, units with more bedrooms typically offer a larger living space, allowing renters to make room for shared furniture or host guests.